The present invention relates to a variable-stroke crank mechanism, and more particularly to a variable-stroke crank mechanism for use in a fluid pressure device, the crank mechanism having a plurality of fluid pressure actuators for relatively rotating a casing and a crankshaft which is rotatably mounted in the casing. The present invention also relates to a plunger-type hydraulic unit which employs such a variable-stroke crank mechanism and can be used as a hydraulic pump, a hydraulic motor, or the like, and to a hydromechanical continuously variable transmission which employs such a plunger-type hydraulic unit.
Fluid pressure device include hydraulic devices which utilize oil as a working fluid, such as hydraulic motors, hydraulic pumps, or the like. Some hydraulic devices comprise a casing, a crankshaft (main shaft) rotatably mounted in the casing, and a plurality of hydraulic actuators disposed substantially radially with respect to the crankshaft, for pressing a crankpin of the crankshaft to rotate the crankshaft and the casing relatively to each other.
When such a hydraulic device is used as a hydraulic motor, the hydraulic actuators are successively operated to produce rotational forces which tend to rotate the crankpin around the crankshaft. When the hydraulic device is used as a hydraulic pump, the crankshaft is rotated about its own axis by a suitable power source to successively operate the hydraulic actuators, thereby pressurizing and discharging the working oil.
Heretofore, the hydraulic actuators and the crankpin are operatively interconnected by a connecting rod assembly as shown in FIG. 16 or 17 of the accompanying drawings.
FIGS. 16 and 17 show conventional connecting rod assemblies 301, 302, respectively. Each of the connecting rod assemblies 301, 302 comprises a main connecting rod 303 having a larger-diameter end 303a rotatably mounted on the crankpin and a plurality of auxiliary connecting rods 305 with ends angularly movably coupled to the end 303a of the main connecting rod 303 through respective coupling pins 304. The connecting rods 303, 305 having opposite ends 303b, 305b angularly movably coupled to the movable members (not shown) of the hydraulic actuators.
The center of the larger-diameter end 303a of the main connecting rod 303 is coaxial with the central axis of the crankpin. In other words, the conventional hydraulic devices having a fixed displacement. Therefore, they suffer the following drawbacks:
When the hydraulic device is used as a hydraulic motor, if the rotational speed of the crankshaft or the output torque of the hydraulic motor is to vary, then the pressure or rate of flow of the working oil to be supplied to the hydraulic actuators should be regulated. To regulate the pressure of the working oil, the output pressure of a compressor which pressurizes the working oil must be regulated. Since the regulation of the compressor output pressure is a complex process, the working oil pressure cannot be regulated in small steps. To regulate the rate of flow of the working oil, it is necessary to have a restriction disposed in the supply passage of the working oil. The restriction, however, increases the resistance imposed to the oil flow by the supply passage.
When the hydraulic device is used as a hydraulic pump, the rate at which the working oil is discharged from the hydraulic pump cannot be adjusted since the hydraulic actuators have a constant stroke. If the pressure under which the working oil is discharged from the pump is to be regulated, the output power of a drive source, such as a motor, which rotates the crankshaft has to be adjusted. As a result, the pressure of the discharged working oil cannot be regulated in small steps.
Hydraulic pumps or motors which incorporate hydraulic actuators operable by the crank mechanism of the type described above are well known in the art as radial- and axial-plunger-type hydraulic pumps or motors. Hydraulic continuously variable transmissions which employ such hydraulic pumps and motors are also well known in the art.
For example, Japanese Laid-Open Patent Publication No. 61(1986)-153057 and U.S. Pat. No. 2,844,002 disclose such hydraulic continuously variable transmissions. The disclosed transmissions comprise axial-plunger-type hydraulic pump and motor as hydraulic units. The pump and the motor are arranged back to back, and have respective pump and motor cylinder casings that are integrally joined to each other. The pump has a pump shaft as an input shaft, and the motor has a motor shaft as an output shaft, the pump and motor shafts being coaxial with each other. When the input shaft is driven to rotate about its own axis, working oil discharged under pressure from the pump is sent to drive the motor, with the motor shaft rotating as the transmission output shaft. A distribution mechanism for transmitting the working oil between the pump and the motor is disposed between the pump and the motor.
The distribution mechanism comprises a distribution housing integrally coupled to the pump and motor cylinder casings, a distribution spool radially slidably disposed in the housing, and a distribution cam for imparting sliding motion to the distribution spool.
In the transmission disclosed in Japanese Laid-Open Patent Publication No. 61(1986)-153057, the distribution cam is a member attached in surrounding relation to the distribution housing and having an inner peripheral surface held against an outer end of the distribution spool. The inner peripheral surface of the distribution cam is positioned eccentrically with respect to the axes of the pump and motor cylinder casings. A mechanism having such distribution cam will hereinafter be referred to as an outer cam mechanism. When the pump and motor cylinder casings are rotated to rotate the distribution housing therewith, the distribution spool also rotates with the distribution housing, and is guided by abutment against the inner peripheral surface of the distribution cam so that the distribution spool is reciprocally moved radially by a distance which is the same as the distance by which the inner peripheral surface of the distribution cam is eccentric with respect to the axes of the pump and motor cylinder casings.
An oil passage communicating with cylinder bores in the pump cylinder casing and an oil passage communicating with cylinder bores in the motor cylinder casing are defined in the distribution housing. When the distribution spool is reciprocally moved radially, those cylinder bores in the pump cylinder casing which are in a compression stroke and those cylinder bores in the motor cylinder casing which are in an expansion stroke communicate with each other, and those cylinder bores in the pump cylinder casing which are in an expansion stroke and those cylinder bores in the motor cylinder casing which are in a compression stroke communicate with each other. Therefore, the working oil that is discharged from the pump is sent to the motor, rotating the motor shaft, and is then returned to the inlet port of the pump.
In the transmission disclosed in U.S. Pat. No. 2,844,002, the distribution cam is a ring-shaped member attached to the distal end of a support member projecting into the motor in surrounding relation to the motor shaft, the ring-shaped member being disposed eccentrically with respect to the motor shaft. The ring-shaped member has a outer peripheral surface held against an inner end of the distribution spool. A mechanism having such distribution cam will hereinafter be referred to as an inner cam mechanism. Upon rotation of the distribution housing, the distribution spool is also reciprocally moved radially by a distance which is the same as the distance by which the the distribution cam is eccentric with respect to the motor shaft.
With the inner cam mechanism employed, it is necessary to provide a fixed support member which supports the inner cam mechanism, the fixed support member projecting into the motor in surrounding relation to the motor shaft. Therefore, the entire structure is complex. According to the disclosed arrangement, the inner cam mechanism is fixedly supported by the fixed support member and its eccentric position is also fixed. If it is required to rotate the distribution cam to move its eccentric position for reversing the direction of rotation of the motor shaft, then a mechanism for rotating the distribution cam becomes complicated in the case of the inner cam mechanism.
The outer cam mechanism is advantageous in that its structure is simpler because the distribution cam is directly supported by the transmission housing. However, the inner peripheral surface of the distribution cam against which the outer end of the distribution spool is held has a large diameter, and the inner peripheral surface of the distribution cam and the outer end of the distribution spool, which are held against each other, move relatively to each other at a high relative speed. As a result, these abutting surfaces tend to suffer seizure and wear.